Prior to the present invention, as shown by Cizek U.S. Pat. No. 3,383,435, assigned to the same assignee as the present invention and incorporated herein by reference, thermoplastic resin compositions were provided comprising a polyphenylene ether and a styrene resin. Other thermoplastic compositions are shown by Izawa et al., U.S. Pat. Nos. 3,929,930 and 3,929,931, incorporated herein by reference, which are polyphenylene ether having polystyrene grafted onto the backbone. Although these materials in the form of shaped articles have high impact strength, stiffness, good surface appearance, heat resistance and other desirable properties during or after molding, a major shortcoming of these materials as thermoplastics resins is their normally flammable nature. As a result, flame retardant and/or drip retardant agents are commonly incorporated into such blends of polyphenylene ethers and styrene resin prior to molding.
The flammability of normally flammable thermoplastic polymers have been reduced by using antimony-, halogen-, phosphorous- or nitrogen-containing additives commonly referred to as flame retardant agents. For example, aromatic phosphates such as triphenylphosphate or a combination of such compounds with other compounds such as halogenated aromatics have been added as flame retardant agents as shown by Haas, U.S. Pat. No. 3,639,506. Experience has shown, however, that although these phosphate compounds have been found to impart good flame resistant properties to such polyphenylene ether thermoplastic blends, physical properties of the molded blends such as the heat distortion temperature (HDT) are often adversely affected.
In my copending application Ser. No. 651,542, there is taught that the addition of a small amount of an organothiophosphate, for example, a triarylthiophosphate, to polyphenylene oxide-polystyrene resin blends, or grafted copolymers thereof, significantly reduces the flammability of the resulting molded thermoplastic materials. However, it has been found that even though improved flame retardant properties can be achieved by using an effective amount of an organothiophosphate in polyphenylene oxide blends, the resulting flame retardant compositions exhibit a significant degree of stress cracking after heating aging the molded composition. As defined hereinafter, the term "stress cracking" means the formation of small hairline cracks along the edges of a molded thermoplastic part. It results from stresses created due to variations in the cooling rate of different areas of the molded part after the part is released from a mold. In measuring stress cracking, the thermoplastic is molded to a 22".times.73/4".times.2" tub shaped container having 1/8" thick walls. The presence of small cracks in centimeters near the edges is determined by optical microscopy.
As shown by Lohmeijer, U.S. Pat. No. 4,529,761, polyphenylene oxide compositions, having both improved flame retardant and stress cracking resistance, can be obtained by using aromatic phosphates, such as triphenylphosphate, with a mixture of certain alkali metal salts of alkyl sulfonates. Although improved stress cracking can be achieved utilizing the aforementioned combination, the resulting flame retardant polyphenylene oxide composition nevertheless shows some degree of stress cracking. It would be desirable to obtain flame retardant polyphenylene oxide compositions substantially free of stress cracking.
The present invention is based on my discovery that the addition of a small amount of an alkali metal salt of a C.sub.12 to C.sub.18 alkyl sulfonate having the formula, EQU (RSO.sub.3).sub.n M, (1)
where R is an aliphatic radical, and preferably a polymethylene radical having from 12 to 18 carbon atoms, and n is an integer equal to 1 or 2, and when n is 1, M is sodium or potassium, and when n is 2, M is calcium or magnesium. These sulfonate salts, referred to hereinafter as "metal alkyl sulfonate" or "metal alkyl sulfonates" when used in combination with an effective amount of triarylthiophosphate in polyphenylene oxide-styrene blends or graft copolymers thereof can provide high performance thermoplastic molding compositions. The resulting compositions exhibit improved flame retardance and are substantially free of stress cracking as compared to moldable polyphenylene oxide styrene resin compositions of the prior art.